1. Field of the Invention
The present invention relates to heat exchangers and, more particularly, to a thermoelectric cooling device heat exchanger incorporating an improved assembly to remove heat from a cooling fluid without contamination thereof.
2. History of the Prior Art
The development of thermoelectric cooling (TEC) devices has revolutionize the cooling industry. Conventional cooling has typically required the expansion and compression of gases, such as Chloroflorocarbons (CFC's) and Hydrochloroflorocarbons (HCFC's) to effect the absorption of heat for lowering the temperature of a fluid flowing in association therewith. Unfortunately, these substances are damaging to the earth's ozone layer and are being phased out in future years. The development of solid state cooling systems in the form of TEC devices, has permitted small commercial packages to be developed that are capable of precise temperature control in a variety of applications where environmental concern, size, weight, performance, and noise are at issue.
The most typical TEC device incorporates a thermoelectric module/component that utilizes electrical current to absorb heat from one side of the module and dissipate the heat on the opposite side. Heat exchangers are used on each side; the "heat-dissipating" side carrying heat away from the TEC device for maximizing efficiency and the "heat absorbing" side which lowers the temperature of a select material or fluid. If the current direction is reversed, so is the heat pumping. Generally, cold sides and hot sides are developed necessitating an effective means of removing or adding heat from or to a solid, liquid or a gas (typically air).
An example of such an application of a TEC device is seen in U.S. Pat. No. 5,097,829 (the '829 patent) for a temperature controlled cooling system. In this embodiment, the advantages of medically therapeutic cooling of a wound site on a body are discussed. Recent clinical evidence indicates that if the temperature of a body part, particularly a wound site, is lowered a number of therapeutic benefits ensue. First, a lower temperature will reduce swelling and increase the activity of the blood in the wound area to promote healing. Second, a lower temperature at a wound site substantially reduces the pain experienced by the patient. This not only increases the comfort level of the patient but significantly reduces the necessity for the administration of narcotics and other pain medication to the patient's benefit. Third, reduction of the temperature at a wound site increases the flexibility in that region. This is particularly true in the case of a traumatized joint or at the installation site of an artificial joint, where a lower temperature will greatly increase the ability of the patient to exercise the joint. Such treatment can substantially reduce the required period of stay in the hospital.
Initial use of cooling therapy was mainly found in the field of orthopedics. It is now found that post surgical cooling is highly beneficial in the reduction of trauma to the patient. It also increases the rate of healing and reduces the length of a hospital stay. In addition, cooling therapy is also being used in home health care for chronic pain control and to increase joint flexibility and facilitate the rate of healing.
Numerous non TEC prior art devices have been proposed for reducing the temperature of a body part in order to achieve the beneficial results obtained thereby. For example, ice packs have long been used to reduce swelling and achieve some of these benefits. In addition, cold packs containing two chemicals, which when mixed together absorb heat (endothermic reactions), have also been proposed as have cooling pads through which a cooling fluid is circulated and cooled by means of a compressor and refrigerant condensing in evaporator coils. Such devices are very inconvenient and contain many inherent disadvantages.
More recently, devices for circulating a cooling fluid through a blanket applied to a patient have also been proposed. Examples of such structures are shown in U.S. Pat. No. 3,894,213 to Kumar, and U.S. Pat. No. 3,967,627 to Brown, and U.S. Pat. No. 4,459,468 to Bailey. The Bailey patent discloses an apparatus which employs a fluid reservoir for containing a substantial volume of cooling fluid, the temperature of which is regulated by thermal modules. The temperature of the fluid in the reservoir is monitored to maintain a selected temperature. The fluid is pumped from the reservoir through a hose system to a thermal blanket which is applied to the patient and back into the reservoir for further cooling. While such a system has been popular in medical applications, it includes numerous disadvantages. For example, a reservoir system, such as that found in Bailey, requires a substantial pre-cooling time in order to reduce the temperature of the relatively large mass of fluid in the reservoir to a desired temperature level. Secondly, such fluid reservoir type systems must also be primed or go through a priming cycle before use to ensure that there is sufficient fluid in the reservoir before performing the cooling operation. Thirdly, the temperature of the reservoir fluid must be monitored and used as the control parameter. This leads to extreme inaccuracy in attempts to maintain a precise control over the temperature applied directly at the wound site. The heat gained by the fluid between a fluid reservoir and a thermal blanket may often be reflected by a temperature increase as much as 10 to 15 degrees. This results in a very inaccurate regulation of the actual temperature at the wound site.
Another problem associated with the applications of very cold surfaces, such as that of an ice pack, directly to a body part is its effect on the skin. The temperature of the ice pack is very cold and can only be left against the skin for a short period of time. Generally, leaving it longer than 30 minutes can result in damage to the skin. It is much more desirable to be able to apply a temperature in a range between 50 and 55 degrees, which is relatively comfortable to the skin, and maintain that temperature for a substantial number of days. This prolonged application insures that the body part is cooled to the inner depth of the bone or tissue of the traumatized area. With an ice pack, cooling only takes place in the subdural area. In a more precisely controlled temperature application, cooling can take place at a deep penetration for an extended period. Thus, it is highly desirable to be able to maintain precise control of the temperature which is actually contacting the tissue of a wound site and then sustain that temperature for a substantial period of time. In this manner the advantages obtained from the use of cold therapy in a medical application can be vastly increased. This is accomplished with TEC devices as shown in the '829 patent. An improvement is shown in co-pending patent application Ser. No. 08/131,712, from which this application is a continuation-in-part. The heat absorbing heat exchanger for fluid/liquid cooling described in the above-referenced co-pending patent application is critical to the system, as is the heat exchanger for removing thermal energy from the TEC device. Likewise, the type of material utilized in the fluid/liquid heat exchanger may vary. It has been found that aluminum has certain advantages over materials such as stainless steel and the like. Aluminum is light weight, more thermally conductive than stainless steel and is much less expensive. For this reason, it is preferable as a component in the heat absorbing heat exchanger for cooling select fluids.
The use of aluminum as a material for the heat absorbing heat exchanger for fluid cooling does present certain problems. Clear fluid such as deionized water is often used in such cooling systems. Deionized water, when flowing across aluminum, can become contaminated with aluminum molecules, reducing the purity of the water causing it to become more conductive. This is a marked disadvantage in operational systems requiring a high degree of purity. For example, cooling systems of the type described in the above-referenced co-pending application may be used for cooling lasers and the like. The absence of an electrical conductor between the cooler and the laser is therefore important. When the deionized water is contaminated with aluminum molecules, the liquid becomes more electrically conductive and problems may ensue. For this reason, it would be an advantage to improve such systems by preventing aluminum contamination of the cooling fluid.
The present invention provides such an improvement over the prior art by utilizing a teflon coating over the serpentine channels of an aluminum heat absorbing heat exchanger. In this manner, not only is deionized water isolated from the aluminum, but other chemicals that could react with aluminum, such as acid baths, can be pumped through the cooling system. With the utilization of teflon, the problem is virtually eliminated. These advantages can be provided with an improved heat dissipating heat exchanger for the TEC device. The heat dissipating heat exchanger is constructed with a plurality of banks of corrugated foil members sandwiched between adjacent metal plates, the assembly being secured in thermal and mechanical contact with the TEC device by vacuum brazing or the like. The vacuum brazing permits the thermal functioning structure to approach that of a unitary construction whereby thermal gradients across junctures between plates (lamella) and foil and the foil corrugations are substantially reduced. The reduction of thermal gradients, the optimal use of cool air flow and the heat sink surface area improve the heat dissipation capacity of the heat exchanger and provides a compact, low weight heat dissipating heat exchanger in conjunction with the improved design of the heat absorbing heat exchanger.